Causes and consequences of the ocean's biological pump
The production of organic matter at the ocean's surface, its transport, and subsequent remineralization in the ocean interior maintains a vertical carbon and nutrient gradient that sequesters atmospheric CO2, controls the distribution of dissolved oxygen, and modulates the flux of nutrients to the euphotic zone that fuels marine ecosystems. Collectively we refer to the set of processes maintaining these gradients as the biological pump and they are the central focus of this thesis. To advance our understanding of the biological pump, this thesis uses models to analyze and quantify individual processes within the larger biological pump system. Chapter 1 introduces the key processes at question and outlines the specific goals of each research chapter. Chapter 2 uses empirical models and a global database of in-situ observations to examine the environmental drivers of carbon export efficiency, defined as the fraction of organic matter production that is transported from the surface to the interior. We find significant regional variability in the relationship between export efficiency and environmental parameters that are shown to have important consequences for the prediction of global export. Chapter 3 further develops empirical models of carbon export efficiency with a focus on regional export variability in the Southern Ocean. We find new evidence that silica controls Southern Ocean export efficiency via interactions with ocean temperature. Chapter 4 uses a global ocean circulation and biogeochemistry model to examine the sensitivity of the global oxygen distribution to changes in the depth at which sinking organic matter is remineralized. Model experiments demonstrate that raising the remineralization depth of organic matter leads to net oxygen uptake and simultaneous expansion of oxygen minimum zones. Chapter 5 examines the consequences of long term climate-driven changes in the biological pump for the productivity of marine ecosystems and fisheries. The thesis concludes in Chapter 6 with a discussion of the challenges and opportunities in understanding the biological pump in a changing climate, along with my recommendations for follow up research.